On amd64, musl typedefs wchar_t to int, conflicting with -fshort-wchar.
Also, /usr/include is searched before the compiler provided include directory.
As a workaround, use -nostdinc and switch them around.

Add missing `#include <stddef.h>` where wchar_t was defined indirectly by
other standard includefiles.

We need and extra -D for clang, where wchar_t is defined in
/usr/include/allbits.h and gets included by other headers, conflicting with
wchar_t already defined in stddef.h.

See also:	https://bugs.gentoo.org/938012
			https://bugs.gentoo.org/881131
			https://bugs.gentoo.org/832018

--- a/Make.common
+++ b/Make.common
@@ -79,7 +79,7 @@
 #
 
 # ...for both GNU-EFI and TianoCore....
-OPTIMFLAGS      = -Os -fno-strict-aliasing -fno-tree-loop-distribute-patterns
+OPTIMFLAGS      = -Os -fno-strict-aliasing -ffreestanding -nostdinc -isystem $(CPPINCLUDEDIR) -isystem $(EPREFIX)/usr/include $(EXTRACFLAGS)
 CFLAGS          = $(OPTIMFLAGS) -fno-stack-protector -fshort-wchar -Wall
 
 # ...for GNU-EFI....
@@ -168,7 +168,7 @@
   SUBSYSTEM_LDFLAG = -defsym=EFI_SUBSYSTEM=0xa
   LDFLAGS         += --warn-common --no-undefined --fatal-warnings
 
-  ARCH_CFLAGS = -fno-merge-constants -ffreestanding -DEFIAARCH64
+  ARCH_CFLAGS = -fno-merge-constants -ffreestanding -DEFIAARCH64 -nostdinc -isystem $(CPPINCLUDEDIR) -isystem $(EPREFIX)/usr/include $(EXTRACFLAGS)
   ifeq ($(MAKEWITH),TIANO)
     ARCH_CFLAGS += -mcmodel=large -Wno-address -Wno-missing-braces -Wno-array-bounds -ffunction-sections -fdata-sections
   endif
--- a/libeg/lodepng.h
+++ b/libeg/lodepng.h
@@ -32,6 +32,7 @@
 #ifndef LODEPNG_H
 #define LODEPNG_H
 
+#include <stddef.h>
 #include <string.h> /*for size_t*/
 
 // Below block of lines required for GNU-EFI and TianoCore (program hangs
--- a/libeg/nanojpeg.c
+++ b/libeg/nanojpeg.c
@@ -211,6 +211,7 @@
 
 #ifdef  _NJ_EXAMPLE_PROGRAM
 
+#include <stddef.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
@@ -275,6 +276,7 @@
 #endif
 
 #if NJ_USE_LIBC
+    #include <stddef.h>
     #include <stdlib.h>
     #include <string.h>
     #define njAllocMem malloc